This project is designed to provide information about the organization of neuronal systems in the mammalian spinal cord that are involved in the neural control of movement, using the isolated brain stem and spinal cord of neonatal mice in vitro. During FY 1998, we have examined the effect of temperature on low-frequency synaptic depression that is characteristic of primary afferent (dorsal root) monosynaptic excitatory postsynaptic potentials (EPSPs) in lumbosacral spinal motoneurons. Using intracellular recording with conventional sharp micropipettes filled with 2M K acetate and 100 uM QX-314 to block sodium action potentials, we have found that increasing bath temperature from 24 deg C (room temperature) to 32 deg C consistently reduces low-frequency synaptic depression, without concomitant changes in the amplitude of the control EPSPs. Thus the effect is apparently not due to changes in the fraction of transmitter available for immediate release, as found when release probabilities are changed by low external calcium solutions or administration of the GABA-B agonist, Baclofen. Rather it appears to result from some other mechanism. Experimental work on the spinal cord of adult cats has been suspended indefinitely. However, we continue to analyze data tapes obtained in earlier experiments on the modulation of transmission of information through segmental interneurons to motoneurons during fictive locomotion. These data are used to examine the organization of specific sets of last-order interneurons in reflex pathways that project directly to motoneurons, as well as the pattern of control of these pathways by the central pattern generator (CPG) for locomotion, which is located in the spinal cord. We are studying the patterns of control on spinal interneuron systems in order to infer the possible ways in which the locomotor CPG is organized. Our results suggest that the rhythmic output of the locomotor CPG is distributed in space and time to motoneurons through a layer or layers of segmental interneurons, which can be viewed as a pattern distribution system.